In-fibre Bragg gratings (FBGs) have been used extensively as sensors for various parameters including displacement, strain, temperature, pressure, humidity, and radiation dose. FBGs are attractive alternatives to other piezoelectric, resistive or other solid-state sensing technologies because they are small, typically 125 μm in diameter, mechanically compliant, intrinsically robust, chemically inert, resistant to corrosive environments, immune to electromagnetic interference and when suitably configured, are capable of simultaneous multi-parameter sensing.
FBGs have been minimally adapted for medical pressure measurement applications, largely because bare FBGs exhibit low sensitivities to hydrostatic pressures, and their resolution of pressure variations is on the order of MPa. Various mechanical amplification schemes have been developed to increase the sensitivity of FBGs to pressure. Such schemes included application of polymer coatings onto the fibre circumference, or alternatively, onto the pressure diaphragms. The pressure sensitivity of such modified FBGs is increased because the strain caused by a pressure applied to a coated fibre Bragg grating (FBG) is amplified relative to uncoated i.e., bare FBG. Other FBGs modification strategies include housing FBGs in glass-bubbles (e.g., 4-mm diameter). The problem with these types of FBG modification strategies is that because of the increased physical dimensions of coated FBG and housed FBG sensing regions, they do not retain the intrinsic benefits of uncoated FBG technologies, i.e., small size, spatial resolution and mechanical compliance.
The prior art also includes FBG sensors configured for invasive medical assessments, wherein a sensor comprises an optical fibre, a sensing location at which the fibre is configured to provide at least one detectable changeable optical property responsive to a strain within the fibre, and at least one sensing element that undergoes a volumetric change in response to an in-body physical parameter being detected. The sensing element is coupled to the fibre in such a way that volumetric changes induce the strain modulations within the fibre thereby producing detectable variable optical properties. One such sensor employs as a sensing element, a FBG encased in a thick polymer coating such as a hydrogel. The polymer coating amplifies the magnitude of strain on the fibre when the sensor is exposed to a pressure on its outer cylindrical surface. However, due to their increased over-all physical dimensions, coated FBG are more invasive when used for medical applications.